branchcache: use an explicit class for the v2 version
This prepare the introduction of an experimental v3 format version.
In the process, we move the description of the format in that new class.
== New Features ==
* Windows will process hgrc files in %PROGRAMDATA%\Mercurial\hgrc.d.
== New Experimental Features ==
== Bug Fixes ==
* The `indent()` template function was documented to not indent empty lines,
but it still indented the first line even if it was empty. It no longer does
that.
== Backwards Compatibility Changes ==
== Internal API Changes ==
* Matcher instances no longer have a `explicitdir` property. Consider
rewriting your code to use `repo.wvfs.isdir()` and/or
`ctx.hasdir()` instead. Also, the `traversedir` property is now
also called when only `explicitdir` used to be called. That may
mean that you can simply remove the use of `explicitdir` if you
were already using `traversedir`.
* The `revlog.nodemap` object have been merged into the `revlog.index` object.
* `n in revlog.nodemap` becomes `revlog.index.has_node(n)`,
* `revlog.nodemap[n]` becomes `revlog.index.rev(n)`,
* `revlog.nodemap.get(n)` becomes `revlog.index.get_rev(n)`.
* `copies.duplicatecopies()` was renamed to
`copies.graftcopies()`. Its arguments changed from revision numbers
to context objects. It also lost its `repo` and `skip` arguments
(they should no longer be needed).